I am new to ubuntu, I am using ubuntu 13.10, and trying to install NS-2. During the installation I am facing the following error; X11/Xlib.h: No such file or directory Then i tried to install sudo apt-get install libx11-dev This time I am facing the following error; Reading package lists... Done Building dependency tree Reading state information... Done Package libx11-dev is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source E: Package 'libx11-dev' has no installation candidate I have no idea how to deal it, Please help Thanks in advance